The use of Silicon Nitride (SiN) as passivation layer for III-N HEMT devices is known. Passivation is important because of dispersion effects and possible changes in the dynamic behaviour of devices. It has been shown that the passivation layer should be sufficiently thick (for instance having a thickness larger than 200 nm). This is illustrated in FIG. 2. SiN can be deposited in-situ.
A SiN layer can also be used as a gate dielectric if it remains between a barrier and a gate of a transistor type device, as depicted in FIG. 1. Such gate dielectric typically reduces the leakage current of the gate and attributes to an improved reliability of the device.
However, when used as a gate dielectric, the SiN layer should not be too thick: separating the actual gate from the two Dimensional Electron Gas (2 DEG) reduces the capacitive coupling between the two which results in a lower transconductance (gm) of the device and a threshold voltage (Vth) which is negative and too high for practical applications. Ideally a SiN gate dielectric layer is between 3 nm and 10 nm thick.